The present invention relates to voltage biased sections of non-linear transmission lines. The invention is particularly related to, but in no way limited to the use of such non-linear transmission lines in interconnect structures for example, on printed circuit boards, back planes, integrated circuits, and surface mount device (SMD) packages.
Modern large-scale electronics such as telecommunication or computing systems are usually comprised of many interconnected shelves of processing, access and/or memory modules. The processing, access and memory functions are usually provided by modules that plug into a large printed circuit board (PCB) referred to as a backplane (BP) or midplane (MP) located behind the modules. Due to the huge quantities of data available for processing and storage there is a need for ever-increasing data rates on the modules and BPs. With the advent of synchronous optical networks (SONET), telecommunication and computer systems may be linked together by optical fibres that terminate at these modules. Usually, in order to make use of the data carried by the optical fibre, the module must convert the signals to electrical pulses and demultiplex (demux) them to many slower speed data streams. The highest widely used optical data rate is 10 gigabits per second (OC192). Currently, such a data stream is often demuxed by a factor of 16 or even 64 to get 16 622 megabits per second or 64 155 megabits per second streams. This level of demuxinig contributes greatly to the complexity of the electro-optic and processing/switching modules and the routing density on the PCBs. In addition, the power requirements of the demux and mux units can be significant.
The industry has recognized that it is desirable to allow electrical signals at a data rate exceeding 5 or even 10 gigabits per second to be transmitted over standard copper (or other electrical) interconnect. If this could be made possible, elctro-optic modules would not require a demux function at all since the electrical signal would also be at 10 gigabits per second. Furthermore, the routing and processing hardware would be made simpler since only a fraction of the currently existing I/Os and tracks would be needed.
The principal reason that commercially available 10 gigabits per second electrical interconnect has not previously been achieved is that the standard back plane and other electrical connectors possess too high a level of parasitic inductance, capacitance, resistance and conductance. In addition, the PCBs themselves may introduce prohibitive levels of attenuation due to frequency-dependent track resistance and dielectric substrate conductance. These result in attenuation, harmonic distortion and dispersion (to name a few) to an extent that makes sufficiently error-free transmission of data virtually impossible.
Digital electronic systems face many limitations. For example, at the chip level, huge parallel bus structures are required which cause severe routing congestion and high power consumption is involved, especially in cases where inputs and outputs are muxed. Very large, complex packages are being used increasingly and skew management and jitter (pattern dependent and base-line wander) are further problems. At the printed circuit board level similar limitations are faced. For example, thick multi-layer PCBs are increasingly required for high chip input/output levels and as a result these PCBs tend to be expensive and can be unreliable. Above about 20 MHz matched terminations are required and this increases the power consumption. Skew management and jitter are also problems as at the chip level.
It is accordingly an object of the present invention to provide a voltage biased section of non-linear transmission line, which overcomes or at least mitigates one or more of the problems noted above.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.
According to an aspect of the present invention there is provided a voltage biased section of non-linear transmission line comprising:
a plurality of transmission line lengths connected together; and
a plurality of diodes, one diode connected at each end of each transmission line length; and
a voltage applied to the section of non-linear transmission line such that the section of non-linear transmission line is operable over a particular voltage range according to the value of the applied voltage.
This provides the advantage that signal integrity is improved. The section of non-linear transmission line acts to provide pulse edge compression to pulse signals transmitted through it in use and the voltage biasing arranges the section of non-linear transmission line so that it operates on a particular voltage region of the signal.
By improving signal integrity for data transmission rates of about 10 Gbps and above, many advantages are achieved. For example, routing congestion on chips and on printed circuit boards can be reduced and packaging and printed circuit board complexity and cost are also reduced. Importantly, the need for multiplexing between electrical and optical systems is removed as mentioned in the section headed xe2x80x9cbackground of the inventionxe2x80x9d above.
Another advantage of the present invention is that it enables a decrease in power consumption to be achieved. That is, because signal degradation is reduced it is possible to use a smaller voltage swing for the signal and still obtain reasonable data transmission rates.
Multi-level logic systems involve enabling each bit to be in one of more than two different states. For example, each pulse in a signal may have one of four possible states in a particular multi-level logic system. This enables more information to be transmitted per pulse as compared with a binary system. However, much less noise can be tolerated. The present invention enables multi-level logics to be used to further increase throughput because signal integrity is improved.
Preferably said transmission line lengths and said diodes are provided by a distributed diode. This enables the advantages of distributed diodes as opposed to PCB-based lumped non-linear transmission lines to be achieved.
Preferably a first plurality of said transmission line lengths are connected in series to form a first part and a second plurality of said transmission line lengths are connected in series to form a second part; and wherein said first part is arranged to receive a voltage varying input signal and wherein said second part is at ground potential.
Preferably a resistor is connected in series with said voltage such that the distance between the resistor and the first part is less than the distance between the voltage and the first part. This enables stub effects to be reduced.
The distance between the resistor and the first part is also preferably less than about 1 mm, again in order to reduce stub effects.
According to another aspect of the present invention there is provided a non-linear transmission line comprising a plurality of voltage biased sections of non-linear transmission line connected together in series by way of capacitances. For example, the sections of non-linear transmission line may be given different voltage biases in order that each section performs signal compensation on a different voltage region of signal pulses.
Preferably the non-linear transmission line further comprises a pulse conditioning filter connected in series to one of the sections of non-linear transmission line. This provides additional means of compensating for signal compression and enables the number of NLTL sections to be reduced.
Advantageously the non-linear transmission line further comprises a feedback mechanism arranged to adjust the voltage biasing. This enables the compensation provided by the non-linear transmission line to be adjusted.
The invention also encompasses a printed circuit board comprising a non-linear transmission line as defined above; an integrated circuit comprising a non-linear transmission line as defined above; and an individually machine placeable surface mount device (SMD) package comprising a non-linear transmission line as defined above.
According to another aspect of the invention there is provided a signal processing device comprising two non-linear transmission lines as defined above, one of said non-linear transmission lines being arranged to process the rising edge of signal pulses and the other of said non-linear transmission lines being arranged to process the falling edge of signal pulses. This provides the advantage that both the rising and falling edges of signal pulses are compensated.
According to another aspect of the present invention there is provided a method of processing a signal, comprising a succession of pulses, in order to compensate for signal degradation, said method comprising passing the signal through a voltage biased section of non-linear transmission line such that a particular voltage range of the pulses is""subjected to pulse edge compression.
Preferably the method further comprises passing the signal through a plurality of voltage biased sections of non-linear transmission line, each voltage biased section of non-linear transmission line having a different voltage bias, such that a different voltage range of the pulses is subjected to pulse edge compression by each section of non-linear transmission line.
According to another aspect of the present invention there is provided a section of non-linear transmission line comprising a plurality of transmission line lengths connected together; a plurality of diodes, one diode connected at each end of the transmission line length; and an apparatus arranged to apply a voltage to the section of non-linear transmission line, such that in use, when the apparatus applies the voltage, the section of non-linear transmission line is operable over a particular voltage range according to the value of the applied voltage.
The invention is also directed to a method by which the described apparatus operates and including method steps for carrying out every function of the apparatus.
The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.